"The preview also will be available at some time in the not-too-distant future for iOS. Microsoft plans to make Sway previews available for Android and Windows Phone phones in the coming months, as well."

"The preview also will be available at some time in the not-too-distant future for iOS. Microsoft plans to make Sway previews available for Android and Windows Phone phones in the coming months, as well."

Where did you read it was going to IOS first?

On Mashable, Mary Jo Foley was kind enough to not state it clearly :)
Sway will eventually be a universal app available on iOS, Android, Windows Phone and the web but the preview version will roll out to iPhone and the web first.